HIV entry is mediated by the gp41 subunit of the viral envelope protein (Env). This proposal applies current knowledge about the mechanism of gp41 mediated viral entry and folding to select and design viral entry inhibitors and disrupt production of Env. During fusion, gp41 is thought to exist in a transient prehairpin intermediate, which is vulnerable to inhibition. This proposal employs mirror-image phage display to discover small circular D-peptidcs which bind to this intcrmcdiatc and prcvcnt fusion. This approach is promising because 1 ) gp41 is an accessible extraccllular target, 2) Env inhibition could prevent HIV entry and infection rather than slowing viral production after entry (e.g., reverse transcriptase and HIV protease inhibitors), and 3) residues involved in the maturation of the gp41 pre-hairpin intcrmcdiate are strongly conserved among diverse HIV strains, making HIV entry inhibitors less vulnerable to rcsistance mutations. Disruption of intracellular Env folding would render a cell incapablc of producing infectious virus. D-peptides are particularly promising as therapcutic agents since they are not susccptiblc to proteolysis and, therefore, may have supcrior pharmacological properties than L-peptides (orally bioavailability, longer serum half-life). These strategies may be useful for the prevention of HIV infection when the moment of exposure is known (c.g., needle stick injuries in health carc workers) or as a prophylactic agent (c.g., topical cream) to block the initial HIV fusion events at the site of viral cxposure. Phage display tcchniqucs developed in this project will also be broadly applicable to the disruption of other important protein-protein interactions in thc HIV life cycle.